Why Would an Animal Eat Its Own Droppings? Digestion Happens in the Wrong Order

Plant cell walls cannot be digested by any mammal directly and must be broken down by microbes, which takes time and a large chamber to do it in. Cattle and sheep place that chamber before the stomach and small intestine, so everything the microbes release is absorbed afterwards in the normal way. Rabbits, horses and rodents place it after the small intestine instead, in an enlarged pouch at the junction with the large intestine, which means the nutrients and vitamins the microbes produce appear downstream of the part of the gut that could absorb them. Those animals face the choice of losing that output or finding another route to it.

A rabbit produces two distinct kinds of dropping and treats them differently:

• •Hard dry pellets of indigestible fibre, which are discarded
• •Soft moist clusters from the fermentation chamber, which are not
• •The soft kind are taken directly from the animal and swallowed whole
• •They are produced mainly at rest, usually during the day
• •A mucus coat protects the microbes through the stomach
• •They are re-digested in the small intestine, where absorption works

The material recovered is not waste and is nutritionally substantial, which is the point most people find hardest to accept. It carries microbial protein of good quality, built by the bacteria from material the animal could not use. It carries B vitamins and vitamin K, which the microbes manufacture and which the animal cannot. It carries volatile fatty acids from fermentation, which supply energy. Studies preventing the behaviour, by fitting a collar that stops the animal reaching, show clear declines in growth and in the ability to digest fibre, with the effects appearing within weeks, which establishes that the practice is essential rather than merely useful.

The behaviour was overlooked for a long time because it happens at rest and out of sight, and establishing it took deliberate work. Rabbits perform it mostly during the day in the burrow or while hunched with the head tucked under, which looks like grooming and was long recorded as such. Careful observation in the 1930s and 1940s identified the two distinct kinds of dropping and the fact that one is never found in a hutch, and controlled studies followed in which animals were fitted with collars preventing them reaching, with the resulting decline in growth and vitamin status establishing what the behaviour supplies. The finding changed how laboratory and commercial rabbits are housed and fed.

The behaviour is widespread and takes several forms with different reasons behind them. Guinea pigs, chinchillas, hamsters, beavers and many other rodents do essentially what rabbits do. Young elephants, hippos, koalas and pandas eat the droppings of adults, which does something different, since it seeds their gut with the microbes needed to digest their adult diet and is essentially inoculation. Dung beetles and many flies feed on dung as their food source. Dogs eating droppings is a behavioural problem rather than a nutritional strategy in most cases, although poor diet and pancreatic disease can produce it, and the same behaviour in horses is normal in foals.

Fermenting plant material in a chamber placed after the small intestine puts the microbial protein and vitamins downstream of where absorption happens, so the material must pass through again. Rabbits produce soft clusters from that chamber, coated in mucus to protect the microbes, and swallow them whole. Preventing this reduces growth and fibre digestion within weeks, which shows it is essential rather than optional.